1. Field of the Invention
The present invention relates to a metal organic chemical vapor deposition (MOCVD) reactor including a flow guide for preventing recirculation of reactant vapor.
2. Description of the Related Art
The development of thin-film deposition processes for compound semiconductors is crucial to the advancement of sophisticated electronic and optoelectronic devices. High speed electronic transistors, quantum-well diode lasers, light-emitting diodes, photodetectors, and optical modulators are fabricated from structures composed of numerous epitaxial layers (epilayers) that range in thickness from several microns to as thin as a few tenths of a nanometer. These epilayers are deposited, or grown, on a single-crystal substrate whereby under the proper conditions the epilayer replicates the substrate so well that the two are often indistinguishable.
MOCVD, also known as organometallic vapor phase epitaxy (OMVPE), is a highly advantageous thin-film deposition process. Gaseous metal-organic and hydride chemical precursors or reactants are transported by a carrier gas to a hot substrate. Near the substrate surface, the gases pyrolize and deposition occurs by the recombination of atomic or molecular species. The thickness and composition of the epilayers that are formed can be controlled by adjusting various parameters such as the concentration of reactants, carrier-gas flow rate, reactor pressure, and growth time. MOCVD may be advantageously utilized for the formation of thin films of gallium arsenide (GaAs), aluminum gallium arsenide (AlGaAs), cadmium telluride (CdTe), and mercury cadmium telluride (HgCdTe).
MOCVD reactors are available in various configurations, including horizontal, vertical, and barrel. In a vertical reactor to which the present invention specifically relates, an object for deposition is mounted on a heated, rotating susceptor disk. A metal-organic vapor including chemical reactants suspended in an inert gaseous carrier such as hydrogen is generated and discharged into the upper portion of the reactor, and flows downwardly through a cylindrical shroud or passageway toward the rotating disk. In order to sustain the flow of vapor, a radial space is provided between the periphery of the disk and the inner wall of the passageway, and the lower portion of the reactor is subjected to reduced pressure. Thus, the portion of the vapor which is not deposited on the object flows downwardly through the radial space and out the reactor through the lower portion thereof.
The MOCVD process, however, has suffered from the drawback that precise control of film thickness, composition, and doping over large areas has heretofore been unattainable. Such control is crucial because advanced circuit concepts require the fabrication of multilayer structures that are highly uniform over large substrate areas.
A major cause of this and other problems in MOCVD is that under certain operating conditions, a large recirculation cell or vortex develops along the wall of the reactor and causes crystal structure and surface morphology problems. In the presence of recirculation, the MOCVD reactor cannot change the reactants quickly enough to obtain an abrupt interface between different types of epilayers such as n-type and p-type. Recirculation suspends reactants and doping constituents in the recirculation vortex above the growth surface after the flow of the reactants is shut off. The constituents are then present to deposit on the crystal substrate surface in a slowly decreasing concentration, resulting in a gradual interface change rather than the desired abrupt interface change. Defects form in the crystal around the precipitates, resulting in poor crystal structure and resulting poor performance of devices fabricated in the MOCVD epilayers.